This invention relates to the use of fluorinated polyurethanes, endowed with a particular combination of properties, for the treatment of films, sheets and similar thin structures made of natural materials such as cellulose, graphite, mica, leather and kaolin.
Fluorinated polyurethanes containing polyoxyperfluoro alkylenic blocks alternating with hydrogenated rigid segments are disclosed in Italian Patent No. 903,446, U.S. Pat. No. 4,782,130 and European Patent Applications No. 359,272 and 359,273.
Said polyurethanes are used as elastomers for tightness systems employed in very hard operating conditions, characterized by an aggressive environment and with very low temperatures.
The principal aim of the above Italian Patent No. 903,446 is the synthesis of perfluoropolyether prepolymers containing hydroxyl terminal groups for the preparation of cross-linked polyurethanes, endowed with a very low Tg. Suitable polyisocyanates are the conventional hydrogenated aliphatic, or aromatic ones and the fluorinated aliphatic ones.
When the above said isocyanates are employed, problems of homogeneity during the synthesis are often encountered. Moreover, the structures disclosed in the above Italian Patent do not allow to obtain products endowed with sufficient mechanical properties. In particular, the tensile strength is usually less than 1 MPa.
The aim of the other three above mentioned documents is the obtainment of materials having improved mechanical properties. This is obtained by means of a block structure constituted of flexible blocks of perfluoropolyether chains and rigid segments constituted of a hydrogenated diol or polyol, an hydrogenated diisocyanate and, optionally, an hydrogenated diamine.
The thus obtained polyurethanes are elastomers having a high elastic modulus and, therefore, endowed with a quite high rigidity, which is useful in some application but undesired in others. In the preparation of these polyurethanes, in view of the contemporary presence of fluorinated prepolymers and hydrogenated monomers having low reciprocal compatibilities, it is necessary to use mixtures of particular solvents, one of which is a solvent of the fluorinated substances and the other of the hydrogenated ones.